Harsh conditions make environmental monitoring a very challenging task. Sensors deployed for environmental monitoring must adhere to stringent quality requirements to ensure reliable data output. The ocean is an example of an environment in which salinity, temperature and pressure conditions result in a corrosive medium, making the task of environmental monitoring increasingly difficult.
Conductivity, temperature and depth (CTD) data of the ocean are important parameters for oceanographic research applications and are used to determine salinity of the ocean water. Conventional methods of measuring conductivity known in the art involve the immersion of two metal electrodes in a fluid whose conductivity is to be measured. A known current is then applied to one of the immersed electrodes and the resulting voltage is measured. The resistive loss as the current passes through the fluid is measured and converted to the corresponding conductivity reading.
One of the main problems associated with this prior art method is that corrosion and fouling of the metal electrodes in contact with the fluid commonly occurs. In an effort to eliminate the corrosion and fouling problems associated with this method, inductive type conductivity sensors have been introduced to the art. With this method, insulated toroidal coils are used to inductively couple an alternating signal through the fluid. The first coil is connected to a frequency oscillator, which induces a magnetic field within the coil. This field couples through the fluid and induces a current in the second coil. The voltage measured at the second coil is compared against a reference value to determine the voltage drop through the fluid. This value is then used to calculate the fluid conductivity as in the conventional design. The toroidal type design which employs insulated coils overcomes the problem of fouling, making it useful for corrosive environments like sea water.
Additionally, toroidal inductors are used in a number of RF (radio frequency) applications where good magnetic shielding is desirable. Conventional toroids, using a ferrite core, are also preferred in circuits that need high power handling capability, inductance and Q factor. The Q factor, or quality factor, of an inductor is the ratio of its inductance to its resistance at a given frequency, and is a measure of its efficiency. The higher the Q factor of the inductor, the closer it approaches the behavior of an ideal, lossless, inductor. Furthermore, the low stray-field intensity of toroidal inductors allows them to be placed in close proximity to other circuitry with low levels of parasitic cross-talk. The primary performance limitation of a ferrite core toroid is the loss due to the induction of eddy current. The loss due to eddy current can be reduced by proper selection of the core material, shape and turn diameter.
Miniaturization of electronic circuits is a goal in virtually every field, not only to achieve compactness in mechanical packaging, but also to decrease the cost of manufacture of the circuits. Many digital and analog circuits, including complex microprocessors and operational amplifiers, have been successfully implemented in silicon based integrated circuits (ICs). These circuits typically include active devices such as bipolar transistors and field effect transistors (FETs), diodes of various types, and passive devices such as resistors and capacitors.
One area that remains a challenge to miniaturize are radio frequency (RF) circuits, such as those used in cellular telephones, wireless modems, and other types of communication equipment. The problem is the difficulty in producing a good inductor in silicon technologies that is suitable for RF applications. Attempts to integrate inductors into silicon technologies have yielded either inductor Q values less than five or required special metallization layers such as gold.
It is well known that the direct current (DC) resistance of a metal line that forms a spiral inductor is a major contributor to the inductor Q degradation. One way to reduce this effect is to use wide metal line-widths, however, this increases the inductor area and the parasitic capacitance associated with the structure. The larger inductor area limits the miniaturization that can be achieved, and the parasitic capacitance associated with the larger area decreases the self-resonance frequency of the inductor, thereby limiting its useful frequency range. Also, since the Q is directly proportional to frequency and inversely proportional to the series loss of the inductor, the metal line widths cannot be chosen arbitrarily large.
There exists a need in the art for a miniaturized corrosion resistant conductivity sensor that can be easily packaged and fitted to a buoy and deployed in underwater applications and that has low power consumption.